The present invention relates to the process of recausticizing kraft liquor and more specifically to the production of sodium polysulfide and sodium hydroxide from the oxidation of white liquor.
In the conventional kraft process for the production of wood pulp, wood chips are digested in an alkaline cooking liquor called white liquor. The active components of this liquor are sodium hydroxide and sodium sulfide. Although sodium sulfide hydrolyses to sodium hydrosulfide in white liquor, for the purposes of simplification only sodium sulfide is referred to throughout the text of this specification.
It is known that an increase in carbohydrate yield in a kraft cook can be achieved by the addition of sodium polysulfide to the conventional white liquor. Reference is made to this process in an article published by Svenesk Papperstidn. 49(9):191, 1946 by E. Haegglund. Sodium polysulfide acts as a stabilizing agent of carbohydrates towards alkaline peeling reactions. Thus polysulfide cooking results in a significant pulp yield gain which then provides increased pulp production permitting increased liquor throughput in recovery at the same total solids or thermal load.
One manner of producing polysulfide is to add elemental sulfur to a white liquor which is disclosed in Canadian Patent No. 444,274 to Fuller and Woodside. This approach, however, leads to an imbalance of sodium and sulfur in the kraft recovery process. The end result is a progressive increase in the sulfidity of the white liquor and an eventual increase of sulfur gases emissions. The extra sulfur which is lost in the environment represents a pollution source which is no longer tolerable. An alternative approach to adding sulfur to white liquor for producing polysulfide, is to convert the sodium sulfide already present in the white liquor to polysulfide by an oxidation process. Based on this approach, several processes for the continuous production and recovery of polysulfide have been published. The different methods have been reported by Green, R. P. in Chemical Recovery in the Alkaline Pulping Process, Tappi Press, pp. 257 to 268, 1985 and by Smith, G. C. and Sanders F. W. in U.S. Pat. No. 4,024,229. These procedures generally involve redox, catalytic or electrochemical processes.
One process for converting sodium sulfide is described by Barker in U.S. Pat. No. 3,470,061 relating to the redox formation of polysulfide by treatment of a liquor containing sulfide with an insoluble manganese oxide with a manganese valence greater than two. After oxidation of sulfide to polysulfide by the manganese oxide, the spent manganese oxidant, which is insoluble in the polysulfide liquor, can be physically separated and reused after partial regeneration by oxidation with air. As disclosed by Barker and Ma in Tappi report 56 (5), 112, 1973, the overall oxidation can be formulated as: EQU xNa.sub.2 S+(x-1)MnO.sub.2 +(x-1)H.sub.2 O.fwdarw.Na.sub.2 S.sub.x +(x-1)MnO+(2x-2)NaOH (1)
where x has typically a value of 2.
The regeneration of the manganese oxidant with air can be expressed as: EQU MnO+1/2.sub.2 .fwdarw.MnO.sub.2 ( 2)
The process for the regeneration of manganese oxide, which includes separation, drying and oxidation, has been described by Barker and Ma in U.S. Pat. No. 2,653,824. Because the process requires a large amount of metallic oxidant to produce enough polysulfide to obtain a significant pulp yield increase, an alternative method to regenerate the metallic oxidant has been proposed by Barker and Becker in U.S. Pat. No. 3,860,479. In this last process a clear white liquor is fed in the bottom of a packed tower which contains a catalyst selected from one of the oxides of manganese, iron, cobalt, zinc, aluminum, nickel and chromium or from one of the sulfides of manganese, nickel, copper, iron and cobalt. Simultaneously air or oxygen is also fed in the bottom of the tower and the oxidized liquor is continuously removed from the top of the tower. For a liquor with a sulfide concentration of 30.2 grams of Na.sub.2 S per liter (as Na.sub.2 O), the polysulfide level of the white liquor exiting a laboratory reactor tower ranged between 6.0 and 7.5 grams of polysulfide sulfur per liter for a 17 hour period.
A further process for the production of polysulfide from kraft white liquor oxidation is disclosed by Smith and Sanders in U.S. Pat. No. 4,024,229. The oxidation of white liquor with air or oxygen occurs in the presence of partially wet proofed activated carbon catalyst and is based on the following reactions: EQU 2Na.sub.2 S+O.sub.2 +2H.sub.2 O.fwdarw.2S+4NaOH (3) EQU XS+Na.sub.2 S.fwdarw.Na.sub.2 S.sub.+ 1 (4)
where x has a typical value of 1.
The sulfide and polysulfide may also react with oxygen to produce thiosulfate: EQU 2Na.sub.2 S+2O.sub.2 +H.sub.2 O.fwdarw.Na.sub.2 S.sub.2 O.sub.3 +2NaOH(5) EQU 2Na.sub.2 S.sub.2 +3O.sub.2 .fwdarw.2Na.sub.2 S.sub.2 O.sub.3( 6)
It is presently considered that oxidation of sulfide in white liquor with air or oxygen is very slow without a catalyst and leads mostly to the formation of thiosulfate as shown in reactions 5 and 6, rather than polysulfide as shown in reactions 3 and 4. However, it has been suggested that the presence during oxidation of a granular grade of activated carbon treated with polytetrafluoroethylene to provide wet-proofing over a portion of the surface of the carbon, results in a large increase in the rate of sulfide oxidation and in a substantial formation of polysulfide. The mechanism of action of the hydrophobic surface is not well understood. It is believed, however, that the polysulfide reaction takes place essentially at the solid surface whereas the thiosulfate reaction occurs in the bulk of the sulfide solution. This conception is disclosed by an article in the Paper Trade Journal 159(13), 38-41, May 1 1975., Smith, Knowles and Green.
The process disclosed in U.S. Pat. No. 4,024,229 has a vertical or cylindrical vessel, referred to as a reactor containing the wet-proofed activated carbon catalyst in a fixed bed. The oxidation of white liquor in the reactor is accomplished with compressed air. It has been reported that the lime mud particles dispersed in white liquor, as a result of incomplete separation of the mud from the white liquor in the recausticizing plant of the kraft mill, tend to contaminate the catalyst bed and reduce its activity. Thus, to prevent deactivation of the bed with lime mud, the white liquor pumped to the reactors needs to be extremely clear which usually necessitates the passage of the clarified white liquor through polishing filters placed between the reactor and the white liquor separation unit. The oxidized kraft white liquor from the process, which is sometimes referred to as orange liquor, produces 4.6 g/L of polysulfide sulfur for an initial sulfide concentration of 24 g/L (as Na.sub.2 O) and 9.8 g/L of polysulfide sulfur for initial sulfide concentrations of 48 g/L (as Na.sub.2 O).
A process similar to that disclosed in U.S. Pat. No. 4,024,229, but using a different catalyst is disclosed in Japanese Patent No. 61,259,754. The catalyst in the packed bed is shown to be made of active carbon granules with 0.2 to 4 mm particle size and a macropore volume of 0.25 cc/g of which 35% consists of pores with a diameter greater than 100 nm. However, unlike the catalyst used in U.S. Pat. No. 4,024,229, no wet-proofing treatment of the carbon surface is apparently required to favor polysulfide formation. It has been suggested that oxidation of sulfide is controlled by internal diffusion (see Hara, S. and Ono, T., Japan Tappi Journal 58(1): 46-51 (1988); Ohgushi, Y. and Hara S-I, in "Preprints of the 1988 Spring Conference, CPPA, May 19-21, 1988, Jasper, Alberta, Canada). Initial results have indicated that with this activated carbon catalyst, the polysulfide concentration in the oxidized liquor is 5.7 g/L (as sulfur) for an initial sulfide concentration of 24.4 g/L (as Na.sub.2 O).
For the production of polysulfide, it is apparent that the formation of thiosulfate from reactions 5 and/or 6 is not desirable because thiosulfate is an inert species during cooking. There are, however, other applications in the kraft process where the full oxidation of sulfide to thiosulfate may be desirable.
White liquor can represent an inexpensive source of sodium hydroxide for the purification of flue gases, for oxygen pulping and bleaching processes and for extraction stages of bleaching sequences using chlorine or chlorine dioxide. The major problem with the use of white liquor is the formation of hydrogen sulfide when the pH of the stream falls below 10. Moreover, even when the pH of the process lies above 10, the sulfide in white liquor affects adversely the brightness and the viscosity of the pulp in pulping and bleaching processes using oxygen and in bleaching sequences using chlorine or chlorine dioxide. These problems can be eliminated, however, if the sulfide is entirely converted to thiosulfate in an oxidation stage.
Smith and Sanders in U.S. Pat. No. 4,162,187 describe how the oxidation of white liquor with wet-proofed active carbon catalyst of the process described in U.S. Pat. No. 4,024,229 can be carried out to further oxidize the polysulfide to thiosulfate as shown in reaction 6. A procedure for oxidizing sodium sulfide in white liquor to produce sodium thiosulfate has been disclosed by Hultman, in U.S. Pat. No. 4,053,352. In one embodiment of this invention, the white liquor is oxidized at a temperature within the range of about 50.degree. C. to 130.degree. C. and without the use of catalyst by injecting air into the liquor while maintaining an air flow within the range of about 50 to 150 m.sup.3 /hr.m.sup.2 at standard conditions and with no catalyst. Laboratory tests indicated that the rate of oxidation in small scale equipment was low without a catalyst and that the rate increased with increasing temperature and flow rate. It was also shown that by conducting the test in pilot plant scale equipment, the rate of oxidation increased significantly due to the higher liquid height in the pilot plant reactor.
Thus it has been shown that a number of processes are available to produce polysulfide or thiosulfate from the oxidation with air or oxygen of sulfide in white liquor. However, of all the known processes for polysulfide production, only the method disclosed in U.S. Pat. No. 4,024,229 and a recent modification of this process has reached commercial status. The main disadvantages of this oxidation process are the high capital cost of the equipment, the cost of the carbon catalyst, the progressive deactivation of the catalyst and the frequent acid washes required to reactivate, at least partly, the contaminated catalyst. A simplified process to produce sodium polysulfide or sodium thiosulfate from the oxidation of sodium sulfide is, therefore, required.